1. Field of the Invention
The present invention relates to a method for forming a field oxide film adapted to isolate semiconductor devices formed on a wafer from one another.
2. Description of the Prior Art
In general semiconductor fabrication, devices having the same structure are fabricated in large quantities on a single wafer to perform a special function. For making such devices perform their function, it is necessary to isolate them from one another.
Examples of conventional methods for achieving such an isolation are illustrated in FIGS. 1 and 2. In the drawings, the reference numeral 1 denotes a semiconductor substrate, 2 and 7 oxide films, 3 a polysilicon film, 4 a nitride film, 5 a field oxide region, and 9 polysilicon film spacers.
One of the conventional methods will be described, in conjunction with FIG. 1.
In accordance with the illustrated conventional method, the oxide film 2 and the nitride film 4 are formed in this order over the semiconductor substrate 1. The oxide film 2 and the nitride film 4 are selectively etched to form the field oxide region 5 in which a field oxide film will be formed. For minimizing the size of bird's beak, the polysilicon film spacers 9 are formed on facing side walls of the oxide film 2 and nitride film 4 defining the field oxide region 5, respectively. The polysilicon film spacers serve as a field oxide film for providing a device isolation.
In this method, however, a crystal defect may occur at the semiconductor substrate 1 due to a stress caused by the polysilicon film spacers 9 which are in direct contact with the semiconductor substrate 1. Furthermore, there is a limitation on a reduction in bird's beak size because the field oxide film is produced by a local oxidation of silicon (LOCOS) process.
Another conventional method illustrated in FIG. 2 is adapted to reduce the length of a bird's beak portion of the field oxide film and increase a volume rate of the field oxide film disposed beneath the surface of the semiconductor substrate. This method will be described, in conjunction with FIG. 2.
Over the semiconductor substrate 1, the oxide film 2 for pads and the nitride film 4 are formed in this order. The oxide film 2 and the nitride film 4 are then selectively etched to form the field oxide region 5 through which the semiconductor substrate 1 is partially exposed. A trench having a small depth is formed at the exposed portion of the semiconductor substrate 1. A polysilicon film 3 is then formed over the exposed portion of the semiconductor substrate 1. The resulting structure is then subjected to the same process steps as the method illustrated in FIG. 1.
However, this method also has the problem of crystal defect occurred at the semiconductor substrate because the trench is formed at the semiconductor substrate.